docs(signal/cir): precise diagnosis of remaining ADR-134 P2 rms-spread failure

Diagnosed the one still-ignored CIR test: ISTA emits a spurious ~15-20%-of-
dominant tap at an implausible far delay (~bin 150 / ~3us) that inflates
rms_delay_spread to ~390ns (vs ~53ns true). It sits too close to the real
weakest tap (~30% of dominant) for a safe magnitude cutoff, so the proper fix
is ISTA recovery-quality work (grid de-aliasing / far-tap suppression), not a
band-aid threshold. Sharpened the #[ignore] note accordingly. signal+cir:
470 pass / 0 fail.

Co-Authored-By: claude-flow <ruv@ruv.net>

v2/crates/wifi-densepose-signal/tests/cir_synthetic.rs

@@ -154,6 +154,7 @@ fn save_fixture(path: &str, k_active: usize, csi: &[Complex64], expected_dominan
 }
 
 // ---------------------------------------------------------------------------
+
 // Shared test logic: inject 3-tap channel, run estimator, assert
 // ---------------------------------------------------------------------------
 
@@ -340,7 +341,7 @@ fn should_return_tof_at_40mhz() {
 // ---------------------------------------------------------------------------
 
 #[test]
-#[ignore = "ADR-134 P2 (remaining): RMS delay spread inflated by far ISTA taps above the 1% cutoff; needs delay-window/robust-spread work (dominant-ratio tuning now landed)."]
+#[ignore = "ADR-134 P2 (remaining): ISTA emits a spurious ~15-20%-of-dominant tap at an implausible far delay (~bin 150, ~3us) that inflates rms_delay_spread to ~390ns. Too close to tap2 (~30%) for a safe magnitude cutoff; needs ISTA recovery-quality work (grid de-aliasing / stronger far-tap suppression), not a band-aid. Dominant-ratio tuning landed separately."]
 fn should_produce_positive_rms_delay_spread() {
     let cfg = CirConfig::for_bandwidth_mhz(20);
     let k_active = cfg.delay_bins / 3;